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Патент USA US3037421

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June 5, 1962
H. HACK ET AL
3,037,403
CONTROL. DEVICE FOR UNBALANCE-CORRECTING
MACHINING 0F ROTATING WORKPIECES
Filed Dec. 3, 1958
2 Sheets-Sheet 1
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June 5, 1962
H. HACK ET AL
CONTROL DEVICE FOR UNBALANCE-CORRECTING
Filed Dec. 3, 1958
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3,037,403
MACHINING OF ROTATING WORKPIECES
2 Sheets-Sheet 2
1
United States Patent G‘ 1C@
3,037,403
Patented June 5,‘ 1962
1
2
3,037,403
the workpiece holder, preferably by release of an electric
control effect, thus reliably blocking the workpiece in the
correct angular position for unbalance-compensating op
CONTROL DEVICE FOR UNBALANCE-CORREICT
ING MACHINING 0F ROTATING WORKPIECES
Heinrich Hack and Gunter Jungke, Darmstadt, Germany,
assignors to Carl Schenck Maschinenfabrik G.m.b.H.,
Darmstadt, Germany, a corporation of Germany
Filed Dec. 3, 1958, Ser. No. 778,011
3 Claims. (Cl. 77—5)
eration.
The rotor to be balanced is blocked by the arresting
device, for instance by actuation of a relay, whereafter the
position of the workpiece is accurately ?xed as needed for
performing the unbalance-eliminating operation.
It is among the objects of the present invention to im
The present invention relates to methods and apparatus
prove such controls toward greater accuracy and to mini
for determining unbalance of a rotating body as to mag 10 mize the skill and attention required for performing and
nitude and angular position of such unbalance, and for
supervising the balancing operation.
controlling the machining or other fabricating operation
required for eliminating such unbalance in accordance
with the previously measured unbalance values.
The present application is a joint continuation-in-part
of sole application Serial No. 606,929, ?led August 29,
1956» issued as Patent No. 2,933,984, dated April 26, 1960
in the name of Heinrich Hack.
The above-mentioned application describes a method
Another object of the invention is to afford a com
pletely automatic unbalance-eliminating operation.
It is a further object of the invention to also auto
matically control the above-mentioned machining opera
tion in accordance with the angular position of unbalance
determined by measurement.
To this end, and in accordance with the present inven
tion, we provide a measuring device with electric control
and apparatus for dynamic balancing of rotating work 20 contacts which take care of automatically adjusting the
pieces by means of unbalance-vibration responsive measur
workpiece in the correct angular position for machining,
ing means whose unbalance-angle indicating member is
by also controlling the arresting of the workpiece in that
position.
provided with an electric signal transmitter. The balancing
errors of the workpiece, with reference to one 'or more
The novel features which are considered as character
correction planes, are electrically determined as to their 25 istic of the invention are set forth in particular in the ap
position and also, preferably, as to the magnitude, by a
balancing apparatus comprising a wattmeter, or similar
measuring instrument, and a phase- or angle-reference
pended claims. The invention itself, however, both as to
its construction and its method of operation, together with
additional objects and advantages thereof, will be best
transmitting device.
understood
from the following description of speci?c
Upon completion of the balance measuring operation 30 embodiments when read in connection with the accom
panying drawings, in which:
proper, the signal transmitter controls an arresting or po~
sitioning device which secures the workpiece, during ro
FIG. 1 illustrates schematically a front view of an auto
tation of the latter, in the one proper position needed for
matically operating machine equipped with an arresting
a fabricating tool to remove or add material for correction
device for operation in accordance with the component
of the previously measured valance. This application per 35 measuring method;
tains to that arresting or position controlling device.
FIG. 2 shows, partly in section, a portion of the machine
It is essential for this control device that an arresting
device which can be switched on and off be provided be
located between, and seen in the direction of, the two
arrows II-II indicated in FIG. 1 and showing an alter
tween the phase-angle transmitter and either the rotating 40 nate construction of the latching device;
workpiece or a machine part rotating in a ?xed angular
FIG. 3 shows schematically and partially in section, the
relation to the workpiece. According to the invention,
balancing apparatus of FIG. 1, but with the switching
the arresting device is automatically switched into op
device shown in more detail;
eration by the release of an electric control impulse ef
FIG. 4 is a schematic and perspective view of a watt
fected by the phase-angle transmitter, in order to stop 45 metric control instrument forming a component of the
and arrest the workpiece in the correct angular position
same machine; and
required for the unbalance-correcting operation. The
FIG. 5 shows schematically in plan view individual parts
turning of the workpiece into the correct angular position
required for the correcting machining operation is per
of the measuring and control device of FIG. 3 on a larger
scale.
formed manually by the attendant or by switching on an 50
It will be recognized from the embodiments described
in detail in the above-mentioned parent application Serial
The method and apparatus according to the invention
No. 606,929, that the invention thereof permits placing
described in the parent application are predicated upon
the workpiece into the proper position for balance cor
measuring the unbalance of a rotating body on a balancing
recting operation ‘without requiring any calculation or
machine, as regards angular position and magnitude of
evaluation of the unbalance measuring results by the at
unbalance, with the aid of one or more auxiliary alternat
tendant, and that the rotor is automatically and reliably
ing-current generators to serve as phase transmitter and
arrested in the proper position during the unbalance-elimi
one or more vibration pick-ups for transmitting electric
nating operation. In the embodiments described in the
intelligence indicative of the unbalance to be determined.
above-mentioned application, the slow turning of the ro~
According to a feature of the present invention, we 60 tor to be balanced into the correct position needed for
the unbalance-removing ‘drilling, welding or other opera
provide a controllable arresting device ‘between the angu
electric motor.
larly displaceable stator of the generator, on the one
hand, and the rotor body being processed or a part of a
balancing machine running in synchronism with that body,
tion is effected either manually or by means of an elec
tric motor to be controlled by the attendant until the
tar-resting device becomes clutched in or the proper sig
on the other hand. The arresting device serves to block the 65 nal is issued.
body, hereinafter called “workpiece,” in the proper ma
chining or fabricating position and possesses a member
However, according to another object of
our invention, ‘a signal representing the angular position
and magnitude of unbalance measured may also be auto
matically transmitted to the device for turning the work
that is angularly displaceable in accurately ?xed relation
piece into the proper unbalance-eliminating position, or
to the angular displacement of the generator stator. When
to the device for eliminating the unbalance condition.
this member occupies a position indicative of the un 70 According to another feature of our invention, therefore,
balance angle, it is put into clutching engagement with
the setting of the workpiece to the angular position re
3,037,403
3
4
.
quired for accurate unbalance elimination is automatical
ly effected by one or more measuring instruments which
are equipped with contact arms at the rotating part of
the instrument. According to further features of the in
vention relating to the same object, the adjustment of
the workpiece to the correct angular position for bal
ance correction is carried out during the balance-meas
uring run with the aid of the same drive that rotates the
workpiece during that measuring run; and means are
provided for producing a braking torque which imparts 10
to the drive the slow speed desired for the subsequent
workpiece-setting operation.
According to FIG. 1, the workpiece 101 to be balanced
of switch ‘107, closes a connection of the excitation cir
cuit for the magnet 117.
A latching plate 115 is secured to the end of the car
rier shaft 103 and is so adjusted and rigidly mounted
relative to phase-transmitter 106 and collector switch
107 that, when the electromagnet 117 is energized and
moves the latch pins 113 upwardly, these pins enter into
mating notches of disk 115 and then prevent any further
rotation of shaft 103 and workpiece 101.
An alternate construction of the latching device is il
lustrated in FIG. 3. Instead of the latching plate 115
and pins 1113 of FIG. 2, a perforated arresting disk 117g
is arranged to be lifted by the action of lever 1170 on
plate 117e, as is described in more detail below, and
is mounted on a carrier shaft 103 driven by a belt trans
mission 104 from an electric motor 116. The bearing 15 mates with arresting dogs 117i of disk 117k to prevent fur
ther rotation of the workpiece 101.
assembly 103a of shaft 103 is secured to the machine
base 181 by means of flat springs 180. The bearing
oscillations due to unbalance of the workpiece 101 are
translated into electric voltage by means of an oscilla
tion pickup 1187. The pickup voltages are supplied to 20
pletion of the unbalance analysis, and the subsequent
arresting of the workpiece in the correct angular posi
electric measuring instruments 182a and 182b, preferably
of the wattmetric type, for measuring and, if desired,
workpiece to the arresting position in the embodiments of
For automatic performance of the entire balancing
process, the slow revolution of the workpiece, after com
tion must be mechanized.
Automatic turning of the
FIGS. 1 to 5 is controlled by a switching device in con
for also indicating the unbalance values. Mounted at
junction with an eddy-current brake 185, 186.
the lower end of shaft 103 is an alternating-current gen
The eddy-current brake comprises a copper disk 186
erator 106 (FIGS. 1, 2) to serve as a phase transmitter. 25
mounted on the shaft of drive motor 116 (FIG. 1) and
Generator ‘106 is connected with shaft 103 by a cardanic
an electromagnetic ?eld system equipped wtih a ?eld coil
coupling or universal joint 106a (FIG. 2) so that the
185. The control switch 183 for slow turning of the
rotor shaft 106g of the generator runs in synchronism
workpiece is a two-pole two-position switch. When
with, and in a ?xed phase relation to, the workpiece
carrier shaft 103. The phase transmitter 106 has two 30 switch 183 is in the position “Balance,” it supplies the
drive motor 116 directly with full line-voltage through
stator windings or two stators which are 90° phaseedis
the switch contact 183a. After completion of the measur
placed relative to each other. As a result, the phase
ing run, the selector switch 183 is shifted to the position
transmitter 106 (FIGS. 1, 2) supplies a sine-Wave volt
“Clutch.” This is preferably done automatically by means
age to the measuring instrument 182a and a cosine volt
age to the measuring instrument 182b, or vice versa. 35 of a centrifugal switch 187 (FIG. 2) which responds when
the drive motor 116 is disconnected from the line and
Both measuring instruments are designed as contact
wattmeters as shown in FIG. 4 by way of example.
According to FIG. 4, showing only a portion of one
of the two instruments, the movable instrument system
comprises a moving coil 1820 and a contact arm 182d
rotating together ‘with coil 182a. The Zero position (0)
of arm 18211 is located in the mid-position of the de
?ecting range. Arm 182d carries a contact 132]‘ which
has decelerated to below a given slow speed. The con
tact 183a (FIG. 3) of switch 183 now supplies the motor
116 with reduced voltage from a transformer 184. At
the same time, the switch contact 183:: supplies full line
voltage to coil 185 of the brake magnet which induces
in the copper disk 186 eddy-currents that impart a
counter-torque to the motor.
This secures the desired
of two elongated contact bars 182g and 182k, depend
slow turning speed for the workpiece 101 as required for
reliably arresting the workpiece in the correct angular
ing upon whether arm 182d is de?ected toward the plus
(+) or minus (—) direction from the zero position.
The contact 1182)‘ is further engageable with any one of
101 to be balanced is mounted on a vertical spindle 103
may enter into contact engagement with one or the other
position.
Referring now to FIG. 3 the disk-shaped workpiece
a number of mutually insulated contacts 182i, depend
of the balancing plant and is driven through a belt trans’
ing upon the direction and amount of de?ection. In
stead of having the contact 182]‘ of arm 182:! in con
tinuous sliding contact with the stationary contacts, we
prefer using a design in which the contact 182]‘ is nor
mission 104 from a motor 116. Two mutually identical
?at springs 180 of which only the one in front is visible
in FIG. 3, are fastened to a part 113a of the machine-bed
mally free to move over the stationary contacts without
engaging them and is ‘then intermittently pressed against
the stationary contacts by a setting member or dropping
stirrup as is conventional for such instruments. De
pending upon which of the stationary contacts 182g and
18211 is contacted at a time in the two instruments, these
instruments permit selectively controlling the arresting
device for the workpiece 101 as regards four different
arresting positions relative to the unbalance-eliminating
tools, here consisting of drills 119a and 11% (FIG. 1).
The electric impulses issuing from the instruments 182a
and 18212 are supplied to a commuator switch 107 (FIGS.
1, 2) driven by the phase transmitter 106. The col
lector switch 107 passes the impulses through an elec
tromagnet 117 when the workpiece 101 reaches the cor
rect angular position required for the balance-correcting
operation.
structure 181. These springs support the balancer spindle
103 in oscillatory condition in the known manner. The
spindle oscillations caused by unbalance during rotation
of the workpiece 101 are converted by oscillation pickups
187, for example of the electrodynamic moving-coil type,
into electric voltages which are supplied in known manner
to the moving coils of two wattmetric or similar measur
ing instruments 182a and 18211 located in the cabinet 281.
The connection between pickups and instruments is
schematically represented by a line 219a.
The switching device 107 and an auxiliary generator
106, operating as a phase-angle transmitter, are con
nected with the other end 1031; of the spindle 103, for
example by means of a cardanic coupling or universal
joint 106a so that the switch portion 107:: (FIGS. 3, 5)
and the rotor 106b of the phase transmitter run in syn
chronism with the workpiece 101. During its rotation the
switch segment 107e touches (FIG. 5) four contacts
The commuator switch 107 is mounted on 70
107a to 107d which are in connection with an electro
magnetic arresting device 117, as will be described below.
tween two carbon brushes 107a. A portion of the pe
The phase-angle transmitter issues two alternating volt
riphery of commutator switch 107 is covered ‘by a con
ages 90° phase-displaced from each other (sine-voltage
tact plate i107b of brass or other conducting material
which, during each rotation, for a portion of the cycle 75 and cosine-voltage) of which one reaches the field coils
the shaft 106g of the phase‘transmitter and rotates be
3,037,403
5
6
of measuring instrument 182a Whereas the other is ap
plied to the ?eld coils of a measuring instrument 182b,
the connection being indicated by a lead 106:: (FIG. 3).
Both measuring instruments 182a and 182b (FIG. 3)
ture 11712 downwardly and turns the lever 1170 about
the pivot 117d. The left lever arm raises the ?ange 117e
together ‘with the housing 106d, thus lifting the disk 117g,
guided by a bushing 1171‘ in the bed structure 181, so
that the arresting holes 117h are engaged by the dogs
That is, the moving coil 1820 in the instruments is pro
117i of the disk 117k. The engaged contact 286al2Jr
vided with a pointer-like arm 182d which carries a con
and the contact thus placed under voltage in the series
tact 182]‘ at its tip. During de?ection of the moving
107a—d, indicates the correct angular position of un
coils the contact tip passes over a series of mutually in
balance, so ‘that the arresting device 117 catches the dogs
sulated individual contacts 1821‘. The position of rest of 10 117); in the arresting holes 117a so that the spindle 103
coil 182C and arm 132d is preferably located at about
with the workpiece 101 is arrested in the accurate position
the center or zero point (0) of the entire travel path so
and the too-heavy location of the workpiece becomes
that the de?ections can be distinguished as to their direc
positioned beneath the drills 119a, 11%. The motor 116
tion + or —. Located between the pivot shaft 286a of
then stops and its energizing circuit switches itself off
arm 182a’ and the contacts 1821' are magnetizable rails 15 automatically in known manner. The machining device
182g and 182/1 which are automatically excited, for ex
119a, 11% can now be automatically fed toward the
ample by a timing relay, in given intervals of time and
workpiece, and can be started and controlled. After ter
are designed as contact devices as illustrated in FIG. 4.
which then attract the arm 182d and arrest it in position.
mination of the correcting work, the arresting operation
The contact arm 182d carries electric voltage from a
is terminated by releasing the contact engagement of arm
source x. The contacts 182i from the plus series are 20 182d and by disconnecting the electromagnet 117a. The
conductively connected with the contact 107a of the
disk 117g with the circle of arresting holes now slides
switching device 107. The contacts 182i of the minus
automatically back into the position of rest.
series are connected, for example, with contact N70
It will be obvious to those skilled in the art, upon a
of the switching device 107. Analogously, in the contact
study of this disclosure, that this invention permits of
Wattmeter 132b, the contact series are conductively con 25 various modi?cations and alterations with respect to the
nected with the contact 10712 or 107d of the switching
various machine components ‘and method steps disclosed,
device 107. These connections are indicated by a line
and hence can be embodied in equipment other than as
286g.
The arresting device comprises the electromagnetic
particularly illustrated and described herein, without de
parting from the essential features of the invention and
switch 117a (P16. 3) with an armature 117b to which
within ‘the spirit and scope of the claims annexed hereto.
We claim:
1. Apparatus for correcting unbalance of a rotatable
a double-armed lever 1170 is linked, this lever being
pivoted at point 117d. The lever 117a acts upon a ?ange
117s on the housing 116d of the auxiliary generator 106
and thus can force the perforated arresting disk 117g
against a cam disk 117k ?rmly seated on the balancer
spindle 103.
The drive motor 116 carries a metal disk 186 which
is mounted on the belt sheave appertaining to the trans
mission 104. The disk 186 runs between the two magnet
poles 185 of an eddy-current brake. The brake can be
excited by means of a switch 183 through a transformer‘
184 from the current supply line 183c. Located above
workpiece by a fabricating operation, comprising a drive
for rotating the workpiece, workpiece carrier means and
fabricating tool means of which one is rotatably adjustable
relative to the other about the axis of said carrier means
to place tool and workpiece relative to each other in posi
tion for unbalance correction, balance analyzing means
comprising two wattmetric instruments for indicating mag
nitude and angular position of unbalance respectively,
each of said instruments having a moving coil and station
ary coils, an alternating-current generator having a sine
the workpiece-accommodating means of the balancing ap
wave output circuit and a cosine-wave output circuit and
paratus is an unbalance-correcting machining device rep
having a rotor connected with said drive to rotate in syn
resented by two drills 119a and 11%.
chronism with the workpiece, unbalance-responsive pick
45
The control device according to the invention operates
up means connected with said moving coils, said two out
as follows. When the drive motor 116 is switched on
put circuits being connected with said stationary coils of
for a measuring run of the workpiece 101 located on the
said two respective instruments, said one instrument for
response to angular position of unbalance having means
balancer spindle 193, the above—mentioned (not illus
trated) timing relay is set into operation. Shortly after 50 for arresting its moving coil after de?ection of said moving
the workpiece has reached the normal running speed and
coils, direct-current supply means and alternating-current
after the measuring instruments 182a and 18% have
' supply means, selector switch means connecting said
responded, the timing relay causes the rails 182g and 182th
direct-current supply means with said rotor when‘ in
to 1be magnetized. This forces the contact 1821‘ of arm
switching condition for balance analysis but connecting
182d into the de?ected position, for example into engage 55 said alternatingcturent supply means to said rotor as well
as to said latter moving coil when in switching condition
ment with the contact 286d2+. Thus the contact 182]‘
for balance correction, and relay means connected with
is arrested. The contact of switch 197 which is con
nected with contact 286d2+ transfers the voltage of cur—
said circuits and responsive to occurrence of a given
rent source x to a neighboring contact of switch 107
equalizing voltage in said circuits when said alternating
which is connected with the minus pole through the arm
of the other instrument 18217. Located in this circuit is
the coil 117a of the arresting magnet 117, so that this
current supply means is connected to said rotor and mov
ing coil and when said proper position of tool means and
carrier means is reached, and an arresting device for
securing said proper position, and an eddy current brake
coil now responds. A second timing relay (not illus
connected with said drive motor for causing the latter to
trated) has disconnected the motor 116 in the meantime;
a centrifugal-force switch 183e, mounted on shaft 106a, 65 turn said carrier at creeping speed for operation of said
position control device, said eddy-current brake compris
and connected to coil 183d of switch 183, enters into
ing a copper disc mounted for rotation with the shaft of
operation after the speed of the motor has declined suf
said drive motor, an electromagnetic ?eld system provided
?ciently and then places the switch 183 from the driving
with a brake magnet having a pair of ?eld coils disposed
position 183a into the braking position 133b, This causes
the eddy-current brake 185, 186 to operate; and the motor 70 on opposite sides of said disc for inducing eddy currents
in said disc when said coils are energized so as to impart
116 receives from transformer 184- a current of such
reduced voltage that it moves slowly into the arresting
a counter-torque to said disc in a direction opposed to
the rotational direction imparted to said disc by said mo
position together with the spindle 1G3 and together with
the disk 117k and the arresting dogs 117i. The magnet
tor, a selector switch movable into a ?rst position for
117, excited by the voltage from source x, pulls the arma 75 supplying to said electric motor a predetermined full line
3,037,403
8
said motor, a selector switch movable into a ?rst position
for supplying to said electric motor a predetermined full
line voltage during a balance-measuring run of the appa
motor a reduced voltage subsequent to said measuring
ratus and movable into a second position for supplying
run, centrifugal switch means connected with said motor
shaft and responsive to deceleration of said motor shaft Ur to said motor a reduced voltage subsequent to said meas
uring run, centrifugal ‘switch means connected with said
below a predetermined speed for shifting said selector
motor shaft and responsive to deceleration of said motor
switch to said second position, said selector switch having
shaft below a predetermined speed for shifting said se~
contact means closed in said second position for supplying
lector switch to said second position, said selector switch
voltage to energize said magnet ?eld coils, whereby the
speed of said motor and said carrier is reduced to said 1O having contact means closed in said second position for
voltage during a balance-measuring run of the apparatus
and movable into a second position for supplying to said
supplying voltage to energize said magnet ?eld coils,
creeping speed.
whereby the speed of said motor and said carrier is re
2. Apparatus for correcting unbalance of a rotatable
workpiece by a fabricating operation, comprising a drive
for rotating the workpiece, workpiece carrier means and
fabricating tool means of which one is rotatably ‘adjustable
duced to said creeping speed.
3. In an apparatus for correcting unbalance of a ro
tatable workpiece by a fabricating operation and equipped
with an electric drive motor for rotating the workpiece
about the rotational axis of the workpiece and having a
balance-measuring device provided with a pickup respon
sive to unbalance vibration of the workpiece when the
relative to the other about the axis of said carrier means
to place tool and workpiece relative to each other in
position for unbalance correction, balance analyzing
means comprising two wattmetric instruments for indicat
workpiece is being driven by said drive motor and having
an angularly displaceable indicating member whose an
gular position is indicative of that of the workpiece un
balance being measured, and further equipped with work
ing magnitude and angular position of unbalance respec
tively, each of said instruments having a moving coil and
stationary coils, and alternating-current generator having
a sine-wave output circuit and a cosine-Wave output cir
cuit and having a rotor connected with said drive to rotate
piece carrier means and fabricating tool means of which
in synchronism with the workpiece, unbalance-responsive
25 one is rotatably adjustable relative to the other about the
pickup means connected with said moving coils, said two
output circuits being connected with said stationary coils
axis of said carrier means to place the tool and workpiece
relative to each other in position for unbalance correction,
of said two respective instruments, said one instrument
with a position-control device for arresting said one ro
for response to angular position of unbalance having
means for arresting its moving coil after de?ection of
said moving coils, direct-current supply means and alter
nating-current supply means, selector switch means con
necting said direct-current supply means with said rotor
when in switching condition for balance analysis but con
necting said alternating-current supply means to said rotor
as well as to said latter moving coil when in switching
condition for balance correction, and relay means con
nected with said circuits and responsive to occurrence of
tatable adjusting means in said unbalance-correcting posi
tion; the improvement comprising an eddy-current brake
connected with said drive motor for causing the latter to
turn said carrier means at creeping-speed for operation of
said position-control device, said eddy-current brake com
prising a copper disc mounted for rotation by said drive
motor, an electromagnetic field system provided with a
brake magnet having a ?eld coil for inducing eddy cur
rents in said disc to impart a counter-torque therein in a
direction opposed to the rotational direction imparted to
said disc by said motor, a selector switch movable into
a given equalizing voltage in said circuits when said a1
ternatingcurrent supply means is connected to said rotor 40 a ?r." position for supplying to said electric motor a pre
determined full‘line voltage during a balance-measuring
and moving coil and when said proper position of tool
run of the apparatus and movable into a second position
means and carrier means is reached, and an arresting
for supplying to said motor a reduced voltage subsequent
device for securing said proper position, including speed
to said measuring run, centrifugal switch means con
responsive control means, and circuit means connecting
said latter control means with said arresting device to 45 nected with said motor shaft and responsive to decelera
tion of said motor shaft below a predetermined speed for
prevent operation of said arresting device when the work
shifting said selector switch to said second position, said
piece speed is above a given value, and an eddy current
selector switch having contact means closed in said second
brake connected with said drive motor for causing the
latter to turn said carrier at creeping speed for operation
switch position for supplying voltage to said magnet ?eld
comprising a copper disc mounted for rotation with the
shaft of said drive motor, an electromagnetic ?eld system
provided with a brake magnet having a pair of ?eld coils
disposed on opposite sides of said disc for inducing eddy
means is reduced to said creeping speed.
of said position control device, said eddy-current brake 50 coil, whereby the speed of said motor and said carrier
currents in said disc when said coils are energized so as to 55
impart a counter-torque to said disc in a direction op
posed to the rotational direction imparted to said disc by
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,933,934
Hack _______________ __ Apr. 26, 1960
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